In recent years, cars and the like have been equipped with a car alarm apparatus having a function of sounding an alarm to the surroundings when the possibility of the car being stolen is detected, in addition to a function of sounding a horn (alarm) in the case of providing a warning to peripheral cars or pedestrians at the time of the ordinary use of cars which are moving or the like.
As disclosed in the following Patent Document 1, in this type of car alarm apparatus, it is important that an alarm reliably operates at the time of a theft. FIG. 6 shows a configuration illustrating an electrical circuit in a conventional car alarm apparatus.
This conventional car alarm apparatus has two ordinary horns 81a and 81b, and one exclusive horn 82 operating at the time of a theft. The ordinary horns 81a and 81b are connected to one terminal of a switch 131a included in a first relay circuit 131. The other terminal of the switch 131a is connected to a positive terminal of an electrical source 110 through a first fuse 121. In addition, one terminal of a coil 131b included in the first relay circuit 131 is connected to a first fuse 121 similarly to the switch 131a. The other terminal of the coil 131b is connected to one terminal of a diode 150 and a horn switch 160.
On the other hand, the exclusive horn 82 operating at the time of a theft is connected to one terminal of a switch 132a included in a second relay circuit 132. The other terminal of the switch 132a is connected to a positive terminal of the electrical source 110 through a second fuse 122. In addition, one terminal of a coil 132b included in the second relay circuit 132 is connected to the second fuse 122 similarly to the switch 132a. The other terminal of the coil 132b is connected to one terminal of the diode 150 and a security control unit 170. Meanwhile, as shown in FIG. 6, a system constituted by the first fuse 121, the first relay circuit 131 and the ordinary horns 81a and 81b, and a system constituted by the second fuse 122, the second relay circuit 132 and the exclusive horn 82 operating at the time of a theft are formed in parallel to (separately from) the electrical source 110.
Here, the diode 150 provided between the first relay circuit 131 and the second relay circuit 132 has a connection configuration in which the direction thereof is set so that a current flows from the first relay circuit 131 to the security control unit 170. Thereby, when the electrical conduction is made by an operation of the horn switch 160 (that is, at the time of ordinary use), a current flows to the electrical source 110 through the first fuse 121 and the coil 131b of the first relay circuit 131 to the horn switch 160. A current flows to the coil 131b, whereby the switch 131a of the first relay circuit 131 is closed, and thus a current flows to the ordinary horns 81a and 81b. As a result, the ordinary horns 81a and 81b operate, and an alarm sound is given as an alarm.
In addition, when a theft is detected by a sensor (not shown) or the like, the security control unit 170 is controlled so that the electrical potential of the input end thereof (B point in FIG. 6) becomes equal to the ground potential. Then, a current flows from the electrical source 110 through the first fuse 121, the coil 131b of the first relay circuit 131 and the diode 150 to the security control unit 170, and flows from the electrical source 110 through the second fuse 122 and the coil 132b of the second relay circuit 132 to the security control unit 170. Thereby, the switch 131a of the first relay circuit 131 and the switch 132a of the second relay circuit 132 are simultaneously closed, all the horns of the ordinary horns 81a and 81b and the exclusive horn 82 operate at the time of a theft, and an alarm regarding the theft is issued.
Incidentally, it is often the case that the ordinary horns 81a and 81b are disposed at the front end portion of a car in order to satisfy a sound pressure as an alarm stipulated by law. When a person committing a theft performs machining from the outside of the car on the ordinary horn disposed on this area and grounds an A point in FIG. 6, an excessive current flows in a path that connects the electrical source 110, the first fuse 121, the switch 131a and the A point by the electrical conduction of the switch 131a of the first relay circuit 131 through the control of the security control unit 170, and thus the first fuse 121 is blown. As a result, the ordinary horns 81a and 81b become inoperable. However, when the exclusive horn 82 operating at the time of a theft is disposed at a location that is out of reach from the outside, for example, the engine compartment, the ordinary horn and the exclusive horn operating at the time of a theft become separate from each other. For this reason, even when the above-mentioned A point is grounded, the second fuse 122 is not blown, and the exclusive horn 82 operating at the time of a theft can continue to sound an alarm.